A strategy for evaluating the environmental impact of on-site sanitation systems

In order to ensure that all South Africans are provided with access to adequate sanitation within the constraints of limited national resources, policies currently being considered by the South African government envisage a significant amount of on-site sanitation in use in the urban areas of the country for the foreseeable future. However, concerns exist that widespread use of these systems will cause subsurface migration of contaminants which may have adverse impacts on human health and on the natural environment.

This paper provides an overview of the problem, reviews existing guidelines and presents a more rigorous strategy for evaluating the impact of on-site sanitation on human health and the natural environment.

The suggested strategy, which permits account to be taken of a multitude of variables encountered, is as follows:
- define compliance requirements in terms of both physical location (point of compliance) and allowable contaminant concentration;
- estimate risk of pollution by viruses and bacteria using a 'residence time' approach;
- estimate pollution risk by nitrates using a mass balance approach;
- for both microbiological and chemical contaminants, use a probabilistic approach (as far as the available data allow), allowing appropriate margins of safety in design, such margins of safety still to be determined;
- carry out field monitoring of on-site sanitation schemes (if water resources are to be protected) to provide early warning of contaminant build-up.

The establishment of a set of general principles for compliance requirements together with the application of these principles to the different water bodies (both surface water and groundwater) in South Africa is the most urgent requirement for the implementation of the above strategy.

It is also suggested that evaluation of environmental impact of sanitation systems should not be confined to on-site sanitation alone, but should be extended to all forms of sanitation system, including water-borne sanitation systems as well.